This invention relates to an electro-chemical sensor for measuring carbon monoxide, hydrogen or hydrazine in the surrounding atmosphere, comprising a polarizable, catalytically active measuring electrode which is exposed to the surrounding air and communicates by way of an aqueous acid electrolyte with a non-polarizable atmospheric oxygen electrode as counter electrode.
In immission monitoring, and to some extent also in emission monitoring, it is necessary nowadays to detect traces of gas at extremely low concentrations. This requires measuring processes of the utmost sensitivity. For example, limiting exposure to carbon monoxide is one of the most extensive and urgent problems in many fields or industry. Environmental pollution by carbon monoxide is caused by incomplete combustion, e.g. of engine fuels or domestic fuels. Due to the toxicity and risk of explosion, combined with the impossibility of recongnizing the pollutant by color, odor or taste, monitoring of the concentrations is necessary and this should be as far as possible continuous.
It was therefore an object of the present invention to develop a gas detector to measure the concentrations of harmful substances. The device should be suitable for use by technically unskilled operators and therefore relatively easy to handle, should function according to a simple, reliable method of measuring and should be ready for taking measurements as soon as it is switched on.
Apart from purely chemical and physical measuring methods, electro-chemical methods are frequently used for the determination of carbon monoxide, hydrogen and hydrazine. Measuring cells with 3-electrode arrangements have been proposed for the measurement of carbon monoxide (see. H. W. Bay, K. F. Blurton, H. C. Lieb, H. G. Oswin, International Laboratory, vol. 1, 1972, No. 5, pages 37 to 41; U.S. Pat. Nos. 3,824,167 (1974) 4,013,522 (1977); German Offenlegungsschrift No. 2,155,935 (1972). These are analogous to the usual half cell arrangements used in electro-chemistry, in which carbon monoxide is oxidized in aqueous solutions at noble metal electrodes, mainly platinum, at potentials of +0.9 to 1.5 V against a standard hydrogen electrode, in accordance with the following equation: EQU CO+H.sub.2 .fwdarw.CO.sub.2 +2H.sup.+ +2C
The measuring cell consists of measuring electrode, a counter electrode and a reference electrode. The potential of the measuring electrode with respect to the reference electrode is adjusted by means of an electronic potentiostat (voltage source) and maintained constant within the required range. The carbon monoxide diffusing towards the measuring electrode is oxidized at that electrode and an electric current proportional to the carbon monoxide concentration is produced.
Alternative proposals include measuring cells consisting of 2-electrode arrangements where carbon monoxide is oxidized at the anode and the counter electrode (cathode) either contains an active substance (oxides or mixed oxides of transition elements) whose redox potential polarizes the anode to the desired electrode potential of +0.9 to 1.5 V (see German Offenlegungsschrift No. 23 16 365) or consists of a gas electrode at which atmospheric oxygen, for example, is reduced. The dimensions of the cathode are selected so that the potential of the cathode does not vary with the measuring currents produced.
Arrangements comprising a reference electrode or a counter electrode functioning as a reference point do not operate completely satisfactorily because the reference potential varies over a period of time. This means that the potential of the measuring electrode also changes so that the primary current is liable to increase in the positive or negative direction due to the reduction at atmospheric oxygen or the formation of oxide layers on the surface of the measuring electrode. Since it is virtually impossible to achieve a constant potential when a reference electrode is used, this measuring system is more susceptible to trouble.
In sensors which are used in portable gas warning instruments or in small telemetering equipment, as in the described invention, it is advisable to replace the liquid electrolytes by solid electrolytes or to fix the electrolytes in order to avoid outflow of the acid frequently used as electrolyte. One known method is immobilization simply by suction, e.g. in asbestos powder, quartz powder or a porous organic or inorganic matrix, e.g. porous PVC filter discs or porous glass filter paper. Fine grained ionic exchange resins which absorb aqueous electrolytes may also be used to fix the electrolyte. When such sensor cells are in operation, the volume of the electrolyte is reduced due to loss of water by evaporation, with the result that the contact between electrode and electrolyte can no longer be maintained and the sensitivity of measurement is therefore reduced.